1. Field of the Invention
The present invention relates to vapor compression systems and, more particularly, to a transcritical multi-stage vapor compression system.
2. Description of the Related Art
Vapor compression systems are used in a variety of applications including heat pump, air conditioning, and refrigeration systems. Such systems typically employ working fluids, or refrigerants, that remain below their critical pressure throughout the entire vapor compression cycle. Some vapor compression systems, however, such as those employing carbon dioxide as the refrigerant, typically operate as transcritical systems wherein the refrigerant is compressed to a pressure exceeding its critical pressure and wherein the suction pressure of the refrigerant is less than the critical pressure of the refrigerant. The basic structure of such a system includes a compressor for compressing the refrigerant to a pressure that exceeds its critical pressure. Heat is then removed from the refrigerant in a first heat exchanger, e.g., a gas cooler. The pressure of the refrigerant discharged from the gas cooler is reduced in an expansion device and the low pressure refrigerant then enters a second heat exchanger, e.g., an evaporator, where it absorbs thermal energy before being returned, as a vapor, to the compressor.
The expansion devices employed in such systems are often variable expansion valves that can be adjusted to control the operation of the system. It is also known to combine such variably adjustable expansion valves with a flash tank and a two stage compressor whereby the variably adjustable expansion valves are disposed on the inlet and outlet side of the flash tank. The flash gas tank also includes an economizer line conveying refrigerant vapor from the tank to a point between the two stages of the compressor assembly. The variable expansion valves upstream and downstream of the flash gas tank can be used to regulate the quantity of refrigerant contained within the flash tank and thereby also regulate the pressure within the gas cooler.
One problem associated with use of such variable expansion valves is that they are expensive. Another problem is that they have moving parts and therefore are subject to mechanical failure.
An inexpensive and reliable apparatus for adjusting the efficiency and capacity of a transcritical multi-stage vapor compression system is desirable.